The present invention relates to multilayer films, particularly multilayer films suitable for use as packaging films. The present invention is particularly directed to multilayer films and packaged products using same, wherein the film has a relatively high modulus and a relatively high O2-transmission rate. The present invention is also directed to the use of such films in the packaging of O2-sensitive products, such as lettuce, etc.
Multilayer films have been utilized for the packaging of xe2x80x9coxygen-sensitive productsxe2x80x9d, such as lettuce, i.e., products which exhibit lower shelf life in the presence of either too much oxygen in the package, or too little oxygen in the package. In such multilayer films, the O2-transmission rate, and even the CO2-transmission rate, are of primary importance, especially in the packaging of such O2-sensitive products as vegetables, fruits, and cheese. For example, in the packaging of precut lettuce, the presence of too much O2 in the package results in an enzymatic browning of cut surfaces, known as pink ribbing. On the other hand, if the concentration of O2 in the package is too low, the lettuce tends to spoil due to anaerobiosis.
One of the commercially-available multilayer films which has been used in the packaging of oxygen-sensitive products has an outer heat-resistant layer of an elastomer, such a styrene-butadiene copolymer, and an outer sealant layer of a metallocene-catalyzed ethylene/alpha-olefin copolymer. Although this multilayer film exhibits desired O2 and CO2 transmission rates, as well as a desirable stiffness, this multilayer film has been found to exhibit an undesirable level of xe2x80x9ccurl,xe2x80x9d thereby exhibiting less-than-desired machinability performance. In form-fill-and-seal machinery (xe2x80x9cFFSxe2x80x9d machinery), curl causes film threadup problems as well as film tracking problems. Furthermore, in FFS packaging, the presence of the outer heat-resistant elastomer layer restricts package formation to a fin-type backseal, as the metallocene-catalyzed outer film layer does not seal well to the heat-resistant elastomer layer.
It has also been found that there is a pervasive belief among many skilled in the packaging of a variety of products that a heat seal cannot be made using a film having both outer layers of a low melt point polyethylene-based polymer, i.e., including both polyethylene homopolymer as well as ethylene/alpha-olefin copolymers, as it is believed that the hot bar will tend to stick to the film during sealing.
It would be desirable to provide a film which exhibits desirable O2 and CO2 transmission rates and a desirable stiffness, while reducing or eliminating the curl of the film, as well as providing a film which is suitable to a lap-type backseal for FFS packaging, using constant heat as opposed to impulse heating. Since fogging of the package is also a common problem in the packaging of produce and other food products, it would also be desirable that the film resist fogging, in order to provide the consumer with a clear view of the contents of the package, and in order to provide a more aesthetically appealing package, especially in retail applications where product presentation is important. However, those surface active agents which are effective antifog agents tend to interfere with ink adhesion to the film. This detrimental effect occurs because the antifog agent blooms to the outside surface of the package and interferes with the adhesion of the ink to the film. This detriment is significant for packages designed for consumer end use, as the consumer does not find such a package to be appealing if the ink is smeared or if the ink comes off onto other articles or the consumer. Thus, it would be also desirable to provide the film with an antifog agent on an outer film surface which forms the inside surface of the package, while also providing adequate adhesion for printing on an outer surface of the film, which outer surface serves as the outside surface of the package.
It has been discovered that curl can be eliminated by providing a film with outer layers of ethylene/alpha-olefin copolymer and an inner layer of elastomer, such as styrene/butadiene copolymer. Furthermore, by providing both outer layers of the film with ethylene/alpha-olefin copolymer, a lap-type backseal can be used in FFS type packaging. It has also been discovered that the homogeneous ethylene/alpha-olefin copolymers (e.g., metallocene-catalyzed ethylene/alpha-olefin copolymers) can be provided on one or both outer film layers, in order to provide the film with further enhanced sealing characteristics.
As a first aspect, the present invention pertains to a multilayer film comprising a first outer layer, a second outer layer, and an inner layer. The first outer layer comprises a first ethylene/alpha-olefin copolymer, which, in turn, comprises a homogeneous copolymer. The second outer layer comprises a second ethylene/alpha-olefin copolymer. The inner layer comprises a thermoplastic elastomer. The inner layer is between the first outer layer and the second outer layer, and the inner layer is chemically different from the first outer layer and the second outer layer. The multilayer film has an O2-transmission rate of from about 500 to 50,000 cc/m2 24 hr STP; more preferably, from 1,000 to 20,000 cc/m2 24 hr STP; still more preferably, from about 2,000 to 10,000 cc/m2 24 hr STP. The multilayer film has a modulus of at least 60,000 psi; more preferably, from about 60,000 to 150,000 psi; still more preferably, from about 70,000 to 120,000 psi; and yet still more preferably, from about 80,000 to 100,000 psi.
Preferably, the first outer layer further comprises a surface-active agent component comprising at least one member selected from the group consisting of ester of aliphatic alcohol, polyether, polyhydric alcohol ester of polyhydric aliphatic alcohol, and polyethoxylated aromatic alcohol, wherein the surface-active component is present over the entire outside surface of the first outer layer.
Preferably, the first outer layer has a thickness of from about 0.3 to 0.8 mil, the inner layer has a thickness of from about 0.1 to 1 mil, the second outer layer has a thickness of from about 0.3 to 0.8 mil, and the multilayer film has a total thickness of from about 1 to 3 mils. More preferably, the inner layer has a thickness of from about 0.4 to 0.8 mil.
It has been found that the curl problem can be reduced or eliminated by providing a film having a symmetrical cross-section. That is, by providing a multilayer film having a cross section which is symmetrical in terms of layer arrangement, layer thickness, and layer chemical composition, curl is reduced or eliminated. Of course, some variation, i.e., lack of symmetry, can be present, while still substantially reducing or eliminating the amount of curl which would otherwise result without substantial symmetry. For example, one outer layer can comprise a homogeneous ethylene/alpha-olefin copolymer while the other outer layer comprises a heterogeneous ethylene/alpha-olefin copolymer. One outer layer may also contain an antifog agent, while the other layer does not. Such minor variations in the cross-sectional symmetry of the film have not been found to have a particularly detrimental effect on the curl level exhibited by the film. More severe variations are also possible, while obtaining a relatively low curl level. However, the presence of only two layers, such as a first layer comprising an ethylene/alpha-olefin copolymer and a second layer comprising a styrene/butadiene elastomeric copolymer, has been found to exhibit substantial curl, and it is believed that it is the lack of symmetry of such a film which is the cause of the undesirable level of curl.
Preferably, the homogeneous ethylene/alpha-olefin copolymer in the first outer layer is a first homogeneous ethylene/alpha-olefin copolymer, and the second outer layer comprises a second homogeneous ethylene/alpha-olefin copolymer. More preferably, the first homogeneous ethylene/alpha-olefin copolymer has a density of less than about 0.915 grams per cubic centimeter, and the second homogeneous ethylene/alpha-olefin copolymer has a density of less than about 0.915 grams per cubic centimeter. Still more preferably, the first homogeneous ethylene/alpha-olefin copolymer comprises ethylene mer in an amount of from about 99 to 80 weight percent ethylene, based on copolymer weight, and a first alpha-olefin mer in an amount of from about 1 to 20 weight percent, based on copolymer weight, wherein the first alpha-olefin mer comprises at least one member selected from the group consisting of C4, C6, and C8; and, the second homogeneous ethylene/alpha-olefin copolymer comprises ethylene mer in an amount of from about 99 to 80 weight percent ethylene, based on copolymer weight, and a second alpha-olefin mer in an amount of from about 1 to 20 weight percent, based on copolymer weight, and wherein the second alpha-olefin mer comprises at least one member selected from the group consisting of C4, C6, and C8. Still more preferably, the first homogeneous ethylene/alpha-olefin copolymer comprises ethylene mer in an amount of from about 95 to 85 weight percent, based on copolymer weight, and a first alpha-olefin mer in an amount of from about 5 to 15 weight percent, based on copolymer weight, wherein the first alpha-olefin mer comprises at least one member selected from the group consisting of C8, and a blend of C6 and C4; and the second homogeneous ethylene alpha-olefin copolymer comprises ethylene mer in an amount of from about 95 to 85 weight percent, based on copolymer weight, and a second alpha-olefin mer in an amount of from about 5 to 15 weight percent, based on copolymer weight, wherein the second alpha-olefin mer comprises at least one member selected from the group consisting of C8 and a blend of C6 and C4; and the multilayer film has an O2-transmission rate of from about 2,000 to 10,000 cc/m2/24 hr STP.
Preferably, the inner layer comprises styrene-containing polymer. The inner layer can comprise a blend of styrene homopolymer and styrene/butadiene block copolymer. Preferably, styrene homopolymer is present in an amount of from about 5 to 50 weight percent, based on layer weight; more preferably, from about 10 to 30 weight percent; still more preferably, from about 10 to 20 weight percent; preferably, the balance of the inner layer is styrene/butadiene block copolymer.
The inner layer can be a first inner layer, with the film further comprising a second inner layer and a third inner layer. The second inner layer comprises a first polyolefin and the third inner layer comprises a second polyolefin with the first inner layer being between the second inner layer and the third inner layer. Preferably, the first polyolefin comprises at least one member selected from the group consisting of propylene homopolymer, and propylene/ethylene copolymer containing ethylene mer in an amount of from about 0.1 to 6 weight percent; preferably, the second polyolefin comprises at least one member selected from the group consisting of propylene homopolymer, and propylene/ethylene copolymer containing ethylene mer in an amount of from about 0.1 to 6 weight percent.
Preferably, the inner layer is a first inner layer comprising a first styrene-containing polymer, with the multilayer film further comprising a second inner layer comprising polyolefin, and a third inner layer comprising a second styrene-containing polymer. Preferably, the second inner layer of polyolefin is between the first inner layer and the third inner layer; preferably, the polyolefin comprises at least one member of the group consisting of propylene homopolymer, and propylene/ethylene copolymer containing ethylene mer in an amount of from about 0.1 to 6 weight.
Preferably, the styrene-containing polymer comprises styrene/butadiene copolymer. Preferably, the styrene/butadiene copolymer comprises styrene/butadiene block copolymer comprising butadiene mer in an amount of from about 20 to 40 weight percent, based on block copolymer weight.
As a second aspect, the present invention pertains to a packaged product comprising a product packaged in, i.e., surrounded by, a package comprising a multilayer film according to the present invention. The product comprises at least one member selected from the group consisting of lettuce, cabbage, broccoli green beans, cauliflower, spinach, kale, carrot, onion, pepper, corn, radish, endive, chard, chicory, radicchio, greens, peas, squash, escarole, brussel sprout, mushroom, melon and berry. Preferably, the multilayer film is a preferred multilayer film according to the present invention. In the packaged product according to the present invention, preferably the multilayer film has a tear notch, so that the package is an easy-open package. Preferably, the package is a sealed pouch having two end seals and a backseal connecting the two end seals. The backseal can be a fin seal or a lap seal.
The O2-sensitive product preferably comprises a cut vegetable or fruit comprising at least one member selected from the group consisting of lettuce, cabbage, broccoli, green beans, cauliflower, spinach, kale, carrot, onion, pepper, corn, radish, endive, chard, chicory, radicchio, greens, peas, squash, escarole, brussel sprout, mushroom, melon and berry. Preferably, the O2-sensitive product comprises a cut vegetable comprising at least one member selected from the group consisting of lettuce, cabbage, broccoli, cauliflower, kale, carrot, onion, radish, endive, chard, chicory, radicchio, and escarole, and the film has an O2-transmission rate of from about 3,000 to 8,000 cc/m2/ 24 hr STP.
As a third aspect, the present invention is directed to a packaging process for packaging an O2-sensitive product. The process comprises the steps of (A) forwarding a supply of a multilayer film into a vertical form fill and seal apparatus; (B) passing the film over a collar member of the vertical form fill and seal apparatus, so that substantially vertically-oriented edge portions of the film are adjacent one another, (C) forming a longitudinal seal along at least a segment of the adjacent edge portions of the film, to form a sealed tube segment; (D) collapsing a lower end portion of the sealed tube segment; (E) forming a bottom package seal across the collapsed lower end portion of the sealed tube segment, to form a pouch; (F) adding an appropriate quantity of the O2-sensitive product to the pouch; (G) collapsing an upper end portion of the pouch, and (H) forming a top package seal across the collapsed upper end portion to form a sealed pouch containing the oxygen-sensitive product, whereby a package is formed. The multilayer film is a film according to the multilayer film of the present invention, preferably, a preferred multilayer film according to the present invention. The product is an O2-sensitive product as in the packaged product according to the present invention. Preferably, the vertical form fill and seal machine forms, fills, and seals at least 15 packages per minute, without substantial burn through of the film at the seals. Preferably, the film is sealed at a temperature of from about 70xc2x0 C. to 150xc2x0 C.